A known type of portable bar code reader directs light from a linear array of light emitting diodes [LEDS] onto a printed bar code, and a portion of this light is reflected and directed onto a linear array of charge coupled diodes [CCDS]. The reflected light is received by the CCD array during a first time period and causes a resultant accumulation or build up of electric charge thereon which is monatonically related to the particular code. During a second and immediately successive time period, in the absence of reflected light, the charge on the CCD array is transferred to a shift register which then delivers the resultant electrical date in series form via an amplifier and digitizer to a T L logic output at which the data appears in binary form. The output of the amplifier and digitizer is also fed to an decoder which typically outputs the code as read in ASCII format. The timing and sequence of the two periods is controlled by a timing circuit connected to the CCD array and the shift register.
The light emitted by the LED array is maintained at a relatively constant value so that the total energy of the light reflected onto the CCD array is sufficient to insure proper operation of the CCD array.
However, if, during the first period, the reader position is changed appreciably with respect to the bar code being read, the resultant reading can be blurred or even destroyed. This problem arises because such changes in position changes the pattern of reflected light and adversely affects the pattern and content of the charge built up on the CCD array. Hence the reader must be held in such manner that its position is not changed.
The prior art has recognized the desirability of minimizing the length of time in which the reader position must be held unchanged, since the shorter this length, the easier it is for an operator of the reader to hold it fixed in position. To this end, the prior art solution is to increase the scan rate, thus reducing the length of time in which the reader position must be held constant. However, in order for the reader to function properly when the scan rate is increased, it is necessary to replace the relatively low speed and relatively inexpensive decoders conventionally used by new decoders which function at much higher speeds and which are much more expensive.
The present invention is directed toward another solution for reducing the length of time in which the reader must be held fixed in position which does not use increased scan rates. Consequently, superior results are obtained in a much simpler and less expensive manner.